50 THE DEVONIAN AND CARBONIFEROUS. [bull. 80. 



York by Dr. A. Bruce, in 1809, which lasted a year, and in 1818 the 

 American Journal of Science and Arts was established in New Haven. 



Silliman did not rehearse any detailed account of the state of the 

 science at the time, but gave general statements referring to the p;ist, 

 with comparisons of the general results effected by the American ge- 

 ologists with what had been done by the English and European geolo- 

 gists. 



In 1842 T. A. Conrad published an important paper in the Journal of 

 the Philadelphia Academy of Science, entitled " Observations on the 

 Silurian and Devonian Systems of the United States, with descriptions 

 of new organic remains." A number of fossils were identified, and 

 several points of interest are noted in this paper, indicating the limita- 

 tion of the groups as they were then recognized. The " older Paleozoic 

 rocks" were the equivalent of the Transition of the older nomenclature. 

 The author notes the perfection of the series of rocks in New York 

 State, and "the great convenience they afford for study, in that they 

 lie nearly horizontal." * 



In this paper the " Cambrian rocks" are included in the Silurian, 

 and the Silurian thus includes all the rocks from the Archean upward 

 to the Tully limestone inclusive. Thus it will be seen that the fossils 

 described previous to 1842 as Silurian fossils may have been Silurian 

 or Devonian to the base of the upper Devonian. 



A list of supposed equivalents is given, 2 in which we find the Lower 

 Silurian strata are: " (10) Clinton group, (9) Niagara sandstone, (8) 

 Shales of Salmon River, 3 (7) Blue shale, (5) Trenton limestone, (4) 

 Mohawk limestone, (3) Birdseye limestone, (2) Calciferous limestone, 

 (1) Potsdam limestone." 4 



The " Trenton limestone " is reported as " forming the bed of the Ohio 

 River from Cincinnati to Louisville." 



The Middle Silurian strata are the a Niagara shale, which equals the 

 Wenlock shale, and upward to Oriskany sandstone. 



The Upper Silurian rocks included the lower Ludlow and succeeding 

 rocks upward to the Tully limestone inclusive. 



In the Devonian system, Conrad placed as Lower Devonian the 

 Ithaca group; as Middle Devonian, the Chemung group; as Upper 

 Devonian, the Old Red sandstone. 



The subdivision into Lower, Middle, and Upper Silurian appears to 

 have been original with Conrad, and he proposed the names " Mohawk 

 system" for the Lower, "Helderberg system" for the Middle, and 

 "Onondaga system" for the Upper Silurian groups, respectively, and 

 stated that the systems are based upon the "distinctness of the fossil 

 contents." 



The year 1843 marks one of the most important periods in the history 



1 Jour. Phil. Acad. Sci., 1842, pp. 228-235. 



2 Ibid, p. 230. 



3 These three are regarded as the equivalents of the Caradoc. 



4 Iu his list there is no No. 6, and Conrad states that 7, 5, 4, 3, 2, and 1 are wanting in Europe. 



